1. Field of the Invention
The present invention relates generally to a soft handover method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for determining whether to perform a soft handover.
2. Description of the Related Art
Commonly, a CDMA mobile communication system uses a cell structure in order to support as many users as possible with limited RF (Radio Frequency) resources. That is, the CDMA mobile communication system divides its entire service area into a plurality of small regions, called a “cell,” and separately assigns RF resources to the cells, thereby supporting more users. Here, “cell” refers to a region covered by a specific base transceiver station (BTS), and the cells are identified through unique scrambling codes used by the associated BTSs.
Therefore, in the CDMA mobile communication system, when a mobile station (MS) located in a specific cell moves toward another cell, it is necessary to continue the service even in the new cell through a handover. That is, the “handover” refers to a successive operation of switching a service to a new cell when an MS moves from a region (or cell) of a current BTS in service (hereinafter, referred to as a “source BTS”) to a new BTS (hereinafter, referred to as a “target BTS”). To this end, the MS measures strength of signals received from neighbor cells in order to determine whether a handover is possible. The strength of the received signals can be expressed with Ec/Io [dB].
The handover can be divided into a soft handover and a hard handover. The soft handover is performed in such a manner that when a handover is requested, an MS sets up a new channel to a target BTS in a state where a channel is set up to a source BTS, and then releases one of the two channels at a prescribed point of time. The hard handover is performed in such a manner that when a handover is requested, an MS releases a channel set up to a source BTS and then sets up a new channel to a target BTS. Here, a traffic channel can represent the channel.
The soft handover improves service quality to the MS, but must disadvantageously simultaneously use channels to the source BTS and the target BTS. In contrast, the hard handover may use any one of the channels connected to the source BTS and the target BTS, but cannot disadvantageously guarantee good service quality. Generally, the soft handover is supported by a digital CDMA mobile communication system, and the hard handover is supported by an analog mobile communication system and some digital mobile communication systems. The hard handover is used for a handover between FAs or mobile switching centers (MSCs) in some digital mobile communication systems. Particularly, in an asynchronous mobile communication system, the hard handover is used for data service. Herein, a description will be made with reference to only the soft handover among the two kinds of the handover.
FIG. 1 conceptually illustrates a soft handover performed in a CDMA mobile communication system. Referring to FIG. 1, an MS 114 located in a cell#1 covered by a source BTS 110 moves to a handover region 115. Commonly, the MS 114 simultaneously receives signals from the source BTS 110 and a target BTS 112 neighboring to the source BTS 110, and determines whether a handover is requested, by measuring strength of the two received signals and comparing the measured signal strengths. In FIG. 1, the MS 114 measures strength of the signals received from the source BTS 110 and the target BTS 112 when the MS 114 is located in the handover region 115. However, the MS 114 may measure strength of the signal received from the target BTS 112 even though it is located in the cell#1. Therefore, if the MS 114, as it moves to the handover region, receives a signal with prescribed strength from the target BTS 112, the MS 114 transmits to the source BTS 110 a pilot strength measurement message for requesting a handover. Upon receiving the pilot strength measurement message, the source BTS 110 reports receipt of the pilot strength measurement message to a base station controller (BSC; not shown), so that the BSC performs a soft handover procedure on the MS 114 for a predetermined time period. The BSC determines whether to perform a soft handover by checking prescribed conditions through the soft handover procedure, and transmits a handover direction message to the MS 114 if it is determined that the soft handover is possible. When a handover is completed after a lapse of a prescribed time, the MS 114 transmits a handover completion message to the BSC in reply to the handover direction message. At this point, a new channel set up to the target BTS 112, which enters an active state.
As described above, the MS 114 separately measures strengths of a signal received from the source BTS 110 and a signal received from the target BTS 112, and determines whether to perform a handover based on the measured signal strengths.
FIGS. 2 and 3 illustrate an ideal handover performed in a conventional CDMA mobile communication system. FIGS. 2 and 3 assume strength of a signal received from a source BTS and strength of a signal received from a target BTS in an ideal radio channel environment. That is, FIGS. 2 and 3 exclude a case where the signals received from the source BTS and the target BTS are unstable due to a characteristic of an actual radio channel environment.
Now, a general method for determining by the MS whether to perform a handover based on strengths of the signals received from the source BTS and the target BTS will be described with reference to FIGS. 2 and 3.
First, a first method for determining whether to perform a handover, using a prescribed threshold, will be described with reference to FIG. 2.
Referring to FIG. 2, the MS measures strength P_target of a pilot channel from the target BTS while performing a service with the source BTS. The pilot channel is a channel for transmitting a pilot signal to check a condition of a radio channel. If strength P_target of the pilot channel from the target BTS exceeds a prescribed threshold T_ADD, the MS transmits the pilot channel measurement result to the source BTS through a prescribed message. Thereafter, the MS sets up a traffic channel to the target BTS.
After setting up a traffic channel to the target BTS, the MS measures strength P_source of a signal received from the source BTS. The strength P_source of a signal received from the source BTS can be measured depending on a signal transmitted over the traffic channel. If the strength P_source of the signal received from the source BTS is lower than a prescribed threshold T_DROP, the MS drives a handover drop timer to determine whether a drop time Δt expires. If a state where the strength P_source of the signal received from the source BTS is lower than the prescribed threshold T_DROP continues for the drop time Δt, the MS releases a channel to the source BTS. Therefore, the MS changes the target BTS to a new source BTS, and receives a service from the new source BTS. The MS receives the Δt value from the source BTS through a measurement control message.
The first method determines a handover by comparing strengths of signals received from the source BTS and the target BTS with two thresholds T_ADD and T DROP. However, this method is disadvantageously susceptible to noises.
In order to solve such a problem, a second handover method has been proposed. The second method for determining whether to perform a handover, by comparing strengths of a signal received from a source BTS and a signal from a target BTS, will be described with reference to FIG. 3.
Referring to FIG. 3, the MS measures strengths P_source and P_target of signals received from the source BTS and the target BTS, and calculates a difference, ΔP1=P_source-P_target, between the strengths P_source and P_target of the two received signals. Here, the strength P_target of the signal received from the target BTS can be measured depending on a pilot signal transmitted over a pilot channel. The MS determines whether the ΔP1 satisfies a condition that the ΔP1 is lower than a prescribed reference value (e.g., 2 dB). The reference value becomes a criterion for determining whether to perform a handover. If a state where the calculated ΔP1 satisfies the reference condition (i.e., the ΔP1 is lower than the reference value) continues a predetermined time Δt, the MS transmits the measurement result of a pilot channel from the target BTS to the source BTS through a prescribed message. Thereafter, the MS sets up a traffic channel to the target BTS.
After setting up a traffic channel to the target BTS, the MS measures strengths, P_source and P_target, of signals received from the source BTS and the target BTS, and then calculates a difference, ΔP2=P_target-P_source, between the strengths P_source and P_target of the two received signals. The MS determines whether the ΔP2 satisfies a condition that the ΔP2 is higher than a prescribed reference value (e.g., 4 dB). The reference value becomes a criterion for determining whether to release the traffic channel currently set up to the source BTS in order to end a handover. If the calculated ΔP2 satisfies the reference condition (i.e., the ΔP2 is higher than the reference value), the MS determines whether a prescribed time Δt expires. If a state where the calculated ΔP2 satisfies the reference condition (i.e., the ΔP2 is higher than the reference value) continues for the prescribed time Δt, the MS releases the channel set up to the source BTS. Therefore, the MS changes the target BTS to a new source BTS, and receives a service from the new source BTS. The MS receives the Δt value from the source BTS.
The second handover method has no problem because a handover is normally performed in an ideal radio channel environment or a good radio channel environment. However, a handover may unnecessarily frequently occur in a region having a poor radio-wave condition such as a downtown area with large buildings.
Such examples are illustrated in FIG. 4. That is, FIG. 4 illustrates examples where a handover may unnecessarily occur in a conventional CDMA mobile communication system.
In FIG. 4, there is a case where a difference between strengths of signals received from a source BTS and a target BTS is lower than a prescribed reference value (e.g., 2 dB), as a radio channel environment for the target BTS becomes abruptly better or a radio channel environment for the source BTS becomes abruptly worse. The case where a radio channel environment for the target BTS becomes abruptly better is represented by reference numerals 410, 450, 420, and 430. The case where a radio channel environment for the source BTS becomes abruptly worse is represented by reference numeral 440. Among others, the reference numerals 410, 420, and 430 represent unnecessary handovers. If such a situation occurs, the MS will set up a traffic channel to the target BTS through a handover procedure. In the meantime, if a radio channel environment for the target BTS or the source BTS becomes stable, the traffic channel set up to the target BTS will be released.
When the above-stated conventional handover methods are applied, an unnecessary handover region occurs in the downtown or in a radio channel environment having an unstable radio-wave condition, causing a reduction in capacity of mobile stations that can be serviced by a target BTS. Particularly, in a specific region having a low probability that a normal handover will occur, such as a downtown area, an unnecessary handover operation may occur due to an external factor such as a building.
In such a conventional CDMA mobile communication system, whether to perform a soft handover is determined based on the same condition regardless of whether the radio-wave environment is good or not, so efficiency of MS capacity is undesirably reduced in a region having a poor radio-wave environment.